Radio communication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually, network architectures will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructures and would, therefore, typically prefer to migrate gradually to all IP network architectures in order to allow them to extract sufficient value from their investment in existing infrastructures. Also to provide the capabilities needed to support next generation radio communication applications, while at the same time using legacy infrastructure, network operators could deploy hybrid networks wherein a next generation radio communication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network. Alternatively, a radio communication system can evolve from one generation to the next while still providing backward compatibility for legacy equipment.
One example of such an evolved network is based upon the Universal Mobile Telephone System (UMTS) which is an existing third generation (3G) radio communication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another alternative is the introduction of a new air interface technology within the UMTS framework, e.g., the so-called Long Term Evolution (LTE) technology. Each new generation, or partial generation, of mobile communication systems add complexity and abilities to mobile communication systems and this can be expected to continue with either enhancements to proposed systems or completely new systems in the future.
LTE uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete Fourier transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station, typically referred to as an eNB in LTE, transmits control information indicating to which terminals and on which resource blocks the data is transmitted during the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as the control region is illustrated in FIG. 3.
The interest in deploying low-power nodes, such as pico base stations, home eNodeBs, relays, remote radio heads, etc., for enhancing the macro network performance in terms of the network coverage, capacity and service experience of individual users has been constantly increasing over the last few years. At the same time, there has been realized a need for enhanced interference management techniques to address the arising interference issues caused, for example, by a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks.
In 3GPP, heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adapt to the traffic needs and the environment. However, heterogeneous deployments bring also challenges for which the network has to be prepared to ensure efficient network operation and superior user experience. Some challenges are related to the increased interference in the attempt to increase small cells associated with low-power nodes, also known as cell range expansion.
Regarding interference coordination in heterogeneous networks (HetNets), the so far introduced HetNet solutions for LTE focus on the downlink (DL). The need for enhanced ICIC techniques for DL in such networks is particularly crucial when the cell assignment rule diverges from the Reference Signal Received Power (RSRP)-based approach, e.g. towards pathloss- or pathgain-based approach, sometimes also referred to as the cell range expansion when adopted for cells with a transmit power lower than neighbor cells. The idea of the cell range expansion is illustrated in FIG. 4, where the cell range expansion of a pico cell is implemented by means of a delta-parameter.
To facilitate measurements in the extended cell range, i.e., where high interference is expected, the standard specifies Almost Blank Subframe (ABS) patterns for eNodeBs and restricted measurement patterns for UEs. A pattern is a bit string indicating restricted and unrestricted subframes characterized by a length and periodicity, which are different for FDD and TDD, 40 subframes for FDD and 20, 60 or 70 subframes for TDD.
Restricted measurement subframes are configured to allow the UE to perform measurements in subframes with improved interference conditions, which may be implemented by configuring ABS patterns at eNodeBs, and avoid measuring in high-interference conditions. Restricted measurement patterns are in general UE-specific. Three patterns are currently specified in the standard to enable restricted measurements:                Serving-cell pattern for Radio Link Monitoring (RLM) and RRM measurements;        Neighbor-cell pattern for RRM measurements;        Serving-cell pattern for Channel State Information (CSI) measurements.        
ABS pattern is a transmission pattern at the radio node; it is cell-specific and may be different from the restricted measurement patterns signaled to the UE. In a general case, ABS are low-power and/or low-transmission activity subframes. ABS patterns may be exchanged between eNodeBs via X2, but these patterns are not signalled to the UE, unlike the restricted measurement patterns.
In addition to DL patterns, there may also be defined uplink (UL) patterns e.g. for interference coordination purpose.
So far patterns have been described in relation to interference coordination. However, transmission and measurement patterns may also be used for other purposes, e.g., for energy saving or distributed communication schemes, such as CoMP, DAS system, RRU, RRH, any type of multipoint transmission and/or reception system etc.
Radio Resource Management (RRM) measurements are performed to support RRM the purpose of which is to ensure the efficient use the available radio resources and to provide mechanisms that enable evolved UMTS Terrestrial Radio Access Network (E-UTRAN) to meet radio resource related requirements. In particular, RRM in E-UTRAN provides means to manage, e.g. assign, re-assign and release, radio resources taking into account single and multi-cell aspects. Some example RRM functions are radio bearer control, radio admission control, connection mobility control, dynamic resource allocation and packet scheduling, inter-cell interference coordination (ICIC), some Self-Optimized Networks (SON) functions related to radio resources, and load balancing. RRM may be intra-RAT and inter-RAT, and the measurements may be intra-frequency, inter-frequency and inter-RAT.
The RRM measurements are performed by a node such as a UE, collected and used by the network in a centralized or distributed manner.
The example RRM measurements are:                Radio Link Monitoring (RLM) which is based on out of sync and in sync detection of a serving cell,        Cell identification reporting e.g. E-UTRAN cell search, inter-RAT UTRAN cell search, System Information (SI) acquisition, etc.,        UE transmit power or UE power headroom, e.g. difference between max output power and transmitted power on log scale,        Radio node transmit power, e.g., total or for specific channels or signals,        Any signal strength and signal quality in general,        Interference and pathloss measurements,        Timing measurements        
In LTE, the following timing measurements are standardized in release 9:                1. UE Receive-Transmit (Rx−Tx) time difference,        2. eNodeB Rx−Tx time difference,        3. Timing advance (TA),        4. Reference Signal Time Difference (RSTD),        5. UE Global Navigation Satellite System (GNSS) Timing of Cell Frames for UE positioning,        6. E-UTRAN GNSS Timing of Cell Frames for UE positioning.        
In the above list, 1, 2, 3 are timing-based range measurements, for simplicity, also called herein timing measurements, since they reflect the cell range. These measurements are similar to round trip time (RTT) measurements in earlier systems. These measurements are based on both DL and UL transmissions. In particular, for UE Rx−Tx, the UE measures the difference between the time of reception of a DL transmission that occurs after the UE UL transmission and the time of the UL transmission. For eNodeB Rx−Tx, the eNodeB measures the difference between the time of reception of a UL transmission that occurs after the eNodeB DL transmission and the time of the DL transmission.
In addition in LTE there are timing measurements which are implementation dependent and not explicitly standardized; example is:                One way propagation delay: It is measured by eNode B for estimation of timing advance to be signaled to the UE;        There may be a similar UE measurement in the future.        
The definitions of the timing measurements in the current LTE standard are given below [TS 36.214].
UE Rx−Tx Time Difference:
DefinitionThe UE Rx − Tx time difference is defined asTUE-RX − TUE-TXWhere:TUE-RX is the UE receive timing of downlink radio frame #ifrom the serving cell, defined by the first detected path intime.TUE-TX is the UE transmit timing of uplink radio frame #i.The reference point for the UE Rx − Tx time differencemeasurement shall be the UE antenna connector.ApplicableRRC_CONNECTED intra-frequencyfor
eNB Rx−Tx Time Difference:
DefinitionThe eNB Rx − Tx time difference is defined asTeNB-RX − TeNB-TXWhere:TeNB-RX is the eNB receive timing of uplink radio frame #i,defined by the first detected path in time.The reference point for TeNB-RX shall be the Rx antennaconnector.TeNB-TX is the eNB transmit timing of downlink radio frame#i.The reference point for TeNB-TX shall be the Tx antennaconnector.
Timing Advance (TADV):
DefinitionType1:Timing advance (TADV) type 1 is defined as the timedifferenceTADV = (eNB Rx − Tx time difference) + (UE Rx −Tx time difference),where the eNB Rx − Tx time difference corresponds to thesame UE that reports the UE Rx − Tx time difference.Type2:Timing advance (TADV) type 2 is defined as the timedifferenceTADV = (eNB Rx − Tx time difference),where the eNB Rx − Tx time difference corresponds to areceived uplink radio frame containing PRACH from therespective UE.
Timing measurements may be used for positioning, e.g. with Enhanced Cell Identity (E-CID), Adaptive Enhanced Cell Identity (AECID), pattern matching, hybrid positioning methods, network planning, SON, eICIC and hetnet, e.g., for optimizing the cell ranges of different cell types, configuration of handover parameters, time coordinated scheduling, etc.
Timing advance is also used to control the timing adjustment of UE UL transmissions. The adjustment is transmitted to the UE in the timing advance command. In LTE, for UEs not supporting LTE Positioning Protocol (LPP), the UE timing adjustment is based on TA Type 2 only.
Most (although not all) of the timing measurements are either positioning measurements or may be used for positioning; however, as explained below positioning measurements are not limited to timing measurements only.
At least the following measurements may be used for positioning in LTE:                Reference Signal Time Difference (RSTD) for Observed Time Difference Of Arrival (OTDOA) positioning        Time Of Arrival (TOA) or Time Difference Of Arrival (TDOA) for Uplink Time Difference Of Arrival (UTDOA) positioning (not yet defined for LTE)        Angle of Arrival (AoA) for UL E-CID        RSRP, Reference Symbol Received Quality (RSRQ) for DL E-CID        UE Rx−Tx, eNodeB Rx−Tx and Timing Advance Type 1 and Type 2 for E-CID        UE GNSS Timing of Cell Frames for UE positioning        
The measurements are performed by a measuring node, which may also be a UE, and used for determining the location of the LCS target, which may be a UE or a radio node. Positioning may be UE-based, UE-assisted, or network-based, which determines the node performing radio measurements and the node determining the location. For UE-based positioning, the location is determined by the UE and the measurements are typically also collected by the UE. For network-based positioning or UE-assisted, the location is typically determined by the network, e.g., a positioning node, Evolved Serving Mobile Location Centre, E-SMLC, or Serving Location Centre (SLC) in LTE. For UE-assisted positioning, the measurements are performed by the UE and reported to the network node. For network-based positioning, the measurements are performed by radio network nodes.
It is mandatory for all UEs to support all intra-RAT measurements, i.e. inter-frequency and intra-band measurements, and meet the associated requirements. However the inter-band and inter-RAT measurements are UE capabilities, which are reported to the network during the call setup. The UE supporting certain inter-RAT measurements should meet the corresponding requirements. For example a UE supporting LTE and Wideband Code Division Multiple Access (WCDMA) should support intra-LTE measurements, intra-WCDMA measurements and inter-RAT measurements, i.e. measuring WCDMA when serving cell is LTE and measuring LTE when serving cell is WCDMA. Hence the network can use these capabilities according to its strategy. These capabilities are highly driven by factors such as market demand, cost, typical network deployment scenarios, frequency allocation, etc. Inter-frequency and inter-RAT measurements may be performed for RRM, positioning, SON, MDT, etc.
The UE performs inter-frequency and inter-RAT measurements in measurement gaps. The measurements may be done for various purposes: mobility, positioning, self organizing network (SON), minimization of drive tests etc. Furthermore the same gap pattern may be used for all types of inter-frequency and inter-RAT measurements. Therefore E-UTRAN must provide a single measurement gap pattern with constant gap duration for concurrent monitoring, i.e. cell detection and measurements, of all frequency layers and RATs.
In LTE, measurement gaps are configured by the network to enable measurements on the other LTE frequencies and/or other RATs, e.g. UTRAN, Global System for Mobile communication (GSM), CDMA2000, etc. The gap configuration is signaled to the UE over RRC protocol as part of the measurement configuration. Currently, two measurement gap patterns are defined [TS 36.133]: pattern#0 (40 ms) and pattern#1 (80 ms).
In general, in LTE inter-RAT measurements are typically defined similar to inter-frequency measurements, e.g. they may also require configuring measurement gaps similarly to inter-frequency measurements, but often more measurements restrictions and more relaxed requirements are used for inter-RAT measurements. As a special example there may also be multiple networks, which use the overlapping sets of RATs. The examples of inter-RAT measurements specified currently for LTE are UTRAN Frequency Division Duplex (FDD) Common Pilot Channel (CPICH) Received Signal Code Power (RSCP), UTRA FDD carrier Received Signal Strength Indication (RSSI), UTRA FDD CPICH Ec/No, GSM carrier RSSI, and CDMA2000 1× RTT Pilot Strength.
Inter-band measurement refers to the measurement done by the UE on a target cell on a carrier frequency belonging to a frequency band which is different than that of the serving cell. Both inter-frequency and inter-RAT measurements can be intra-band or inter-band.
The motivation of inter-band measurements is that most of the UEs today support multiple bands even for the same technology. This is driven by the interest from service providers; a single service provider may own carriers in different bands and would like to make efficient use of carriers by performing load balancing on different carriers. A well known example is that of multi-band GSM terminal with 800/900/1800/1900 bands.
Furthermore a UE may also support multiple technologies e.g. GSM, UTRA FDD and E-UTRAN FDD. Since all UTRA and E-UTRA bands are common, the multi-RAT UE may support same bands for all the supported RATs.
A multi-carrier system, or interchangeably called carrier aggregation (CA), allows the UE to simultaneously receive and/or transmit data over more than one carrier frequency. Each carrier frequency is often referred to as a component carrier (CC) or simply a serving cell in the serving sector, more specifically a primary serving cell or secondary serving cell. The multi-carrier concept is used in both High Speed Packet Access (HSPA) and LTE. The UE and radio node need, however, to be configured for CA; otherwise, the carriers are seen as another frequency, i.e., inter-frequency or inter-RAT measurements would apply.
Intra-RAT multi-carrier system means that all the component carriers belong to the same RAT e.g. LTE FDD multi-carrier system, LTE TDD multi-carrier system, UTRAN FDD multi-carrier system, UTRAN TDD multi-carrier system and so on.
In LTE multi-carrier system it is possible to aggregate a different number of component carriers of different bandwidths in the UL and the DL as illustrated in FIG. 5.
In a multi-carrier system one of the component carriers is called the anchor carrier and the remaining ones are called the supplementary carriers. Other terminologies used in literature for the anchor and supplementary carriers are primary and secondary carriers, respectively. Yet other commonly known terminologies for the anchor and supplementary carriers are primary serving cell and secondary serving cell respectively. The primary carrier carries all common and UE-specific control channels. The secondary carrier may contain only necessary signaling information and signals, e.g., those that are UE-specific may be not present in the secondary carrier, since both primary uplink and downlink carriers are typically UE specific. This means that different UEs in a cell may have different downlink primary carriers. This is true also for the uplink primary carriers. The network is able to change the primary carrier of the UE at any time, e.g. for interference coordination or load balancing purpose.
The component carrier may be contiguous or non-contiguous, as shown in FIG. 2. Furthermore in case of non-contiguous carriers, they may belong to the same frequency band or to different frequency bands. A hybrid carrier aggregation scheme comprising contiguous and non-contiguous component carriers are also envisaged in LTE.
A scenario comprising of 5 contiguous component carriers each of 20 MHz (i.e. 5×20 MHz) is being considered for LTE TDD. Similarly for LTE FDD a scenario comprising of 4 contiguous component carriers each of 20 MHz (i.e. 4×20 MHz) in the downlink and 2 contiguous component carriers in the uplink is being studied.
Other carrier types may also be defined in the future.
However, there remain a number of problems associated with standardized approaches to handling measurements and measurement patterns, which problems are discussed in more detail below.